Structures of sheet materials made of asymmetric folds

ABSTRACT

Geometrical structures having alternate asymmetrical folds of drawable or extensible material forming two faces wherein the folds on each of the faces are substantially identical to the other folds on that face. The fold on one of the faces have a rounded profile in cross section which is somewhat wider than the profile on the other face of the structure. The structure may be produced by the rectilinear pleating of a flat sheet or of a longitudinally corrugated sheet or by the chevron pleating of a flat sheet. Such structures may be used as sandwich core materials, filtering screens, cylindrical insulating materials and filtering cartridges, by way of example.

1111110 til tats atent 1 1 Gewiss 1451 A r. 10, 1973 SUQ 11 S @11SlI-WBT "-I. 3,135,174 6/1964 Gewiss 210 493 "11' 0F AS: FQLDS 3,058,59310/1962 Gruner ....210/487 X 2,902,162 9/1959 Humbert, Jr. et al.........210 493 x 1751 Invent Lucie Gem, Vernon 3,115,459 12/1963 Giesse....210/493 x France 3,348,695 10/1967 Rosaen 210/493 X 1 2 796 9896/1957 Kovacs ....210/493X [73] Ass1gnee. Marc M Socaete Anonyme -1 lamafia des 1, 3,543,940 12/1970 Schmidt, Jr ..210/493 X Tecqms EnigmaPrimary Examiner-Reuben Friedman [22] Filed: 20., 1970 AssistantExaminer-F. F. Calvetti [21] A p p1 No; 24,960 Att0rneyLane, Aitken,Dunner & Z1ems Related US. Application Data [57] ABSTRACT [62] Divisionof Ser. No. 541,809, April 11, 1966, Pat, Geometrical structures havingalternate asymmetrical No. 3,550,423. folds of drawable or extensiblematerial forming two faces wherein the folds on each of the faces aresub- [52] US. Cl .210/493, 161/135 stantially identical to the otherfolds on that face. The [51] Int. C1. ..B01d 27/06 fold on one of thefaces have a rounded profile in [58] Field of Search ..2lO/487, 493,497; cross section which is somewhat wider than the profile 137 on theother face of the structure. The structure may be produced by therectilinear pleating of a flat sheet References Cited or of alongitudinally corrugated sheet or by the chevron pleating of a flatsheet. Such structures may UNITED STATES PATENTS be used as sandwichcore materials, filtering screens, 2,826,239 3/1958 Villoresi 161/134 Xcylindrical insulating materials and filtering cartridges, 2,963,12812/1960 Rapp by way of example. 3,025,964 3/1962 Summers et al.3,058,594 10/1962 Hultgren ..210/487 x 9 Claims, 15 Drawing FiguresPATENTE U I 3.726.408

sum 1UF6 'PAIENIEUAPMOIQH. 3.726.408

SHEET 2 [1F 5 STRUCTURES OF SHEET MATERIALS MADE OF ASYMMETRIC FOLDSCROSS-REFERENCE TO RELATED APPLICATION This application is a division ofcopending application Ser. No. 541,809 filed Apr. 11, 1966, now US. Pat.No. 3,550,423.

The present invention is related to novel geometrical structuresexclusively constituted of alternate folds of drawable or extensiblematerials in sheet formation and also to the processes which aresuitable for the making of such structures as well as to their variousapplications.

A structure as per the invention is essentially characterized by thefact that the folds, each one identical to the others which are shownonto one of its two main faces, have a profile somewhat wider than thatof the folds, all of them also identical, which are present on theopposite face. I

The parts of the sheet of such structures which are comprised betweenthe wider folds and the adjoining thinner ones on the opposite face arethe inside walls of the structures, all of them being identical orsymmetrical. The longitudinal section of each wall parallel to thedirection of the folds is a line which can be other than rectilinear aswill be explained hereafter. On the other hand, the cross section ofeach wall from the beginning of one fold to the beginning of the nextone is always a straight line.

The structures of the invention can be fashioned from drawable orextensible sheets of material which are either flat or which initiallyhave longitudinal corru- US. Ser. No. 514,171 filed .lune;9-, 1955,thedisclosures of which are hereby incorporated herein by reference. 1

- In every case, the structures, as per the invention, appear on eachface'to have parallel folds of equal thickness, but-this thickness isdifferent from one face to the other. This asymmetric constitution ischaracterized-in that the wider folds being collected in joining contactonto a first plane, the thinner folds of the opposite face are evenlyspaced onto another-plane which is parallel to the first. The free equalspaces which are providedunder such conditions between the thinner foldshave .for their measure thedifference between the widthof each of thetwo different kinds of folds.

1f, keeping in close contact the wider folds of a limited length ofstructure, lyingflat onto a plane, the

' two ends of this structure are lifted up until the top face of thethinner folds comes into contact, it is clear that the structurehas'taken the shape of a portion of cylindrical annular volume, all thewalls of the incurved structure being evenly distributed in asubstantially radial, fanJike position. The face of the structurehavface. Y

The above remarks show that it is always possible to determine therespective thickness of the two different folds of a structure which isto fill the whole of an annular cylindrical volume of known outside andinside diameters.

A practical process for forming a sheet of drawable or extensiblematerial into a structure having asymmetric folds as per the inventioncomprises making the folding of two such sheets at a time by insertingbetween them a spacer sheet of a layer having a proper thickness andsuch pliability that will allow for nu merous successive foldingswithout showing permanent wear or malformation.

In submitting to an alternate folding the three-ply sandwich thusconstituted (or in chevroning it, if desired), two structures as per theinvention are simultaneously obtained whose folds mated (nested) withthose of the spacing layer are, for each of them, asymmetric, buthowever identical from one structure to the other. Disengagement of thetwo structures thus fashioned from the folds of the median spacing layercan be effected by simply pulling the downstream extremity of thislayer.

The straightening of this inner layer which results fron'l such pullingresults in the ejection, fold after fold, of the two external asymmetricstructures.

When structures having ordinary straight folds are required, thepleating of the three-ply sandwich can be made by any one of thenumerous well known manual or mechanized means which are used forproducing a regular alternate rectilinear folding. i

The pleating of structures having folds of the chevron type can beeffected by making use of any of the chevroning processes which havebeen described by applicants previous patents and patent applications.More particularly, the chevroning can be hand operated between twomother-sheets as taught by US. Pat. No. 3,135,174, or mechanicallyformed by means of any of the machines described in said US. applicationSer. No. 514,171, or French patent 1,197,941 of Jan. 1958 and Frenchpatent of addition No. 74,561 of Nov. 27, 1958, or,U.S. Pat. No.2,950,656, the disclosures of which are hereby incorporated herein byreference.

Here, listed as example, are a few noticeable applications for with thestructures having asymmetric folds as per the invention can be usedeither in their original fla form or wrapped into a cylindricalshape: Il FLAT STRUCTURES a. Sandwich materials The sandwich cores which aremade of sheet materials, either by pleating or by chevroning (see Ser.No. 514,171) are. generally affected byv an uneven repetition of thewalls of their folds. No process can, in fact, give and keep to thesewalls a perfectly equidistant spaclng.

Irregularity in spacing of folds in cores causes a number of the ridgelines of their walls not to meet the inner faces of the sandwich plates.Such walls therefore fail to provide assistance to the total resistanceto crushing of the final product. The present invention, giving thepossibility of keeping in adjoining position the wider folds of one faceof the core and of insuring an accurate spacing of all their walls,offers a satisfactory solution to this problem. Also, the shape(necessarily rounded) of the profile of the flow of a fluid to becleaned, its side offering the (rid of material impurities) under themaximum pressure drop which is acceptable.

On the reverse upstream face of the screen, the

distance between adjoining walls should be the remainder of theavailable spacing obviously more important.

This is leaving larger free spaces for collectihg the impurities at thebottom of the recesses after theireasy interception on adjacent walls.

Larger available spaces between wallson the. upstream face of the screencause the persistence of a free passage provided to the flow.to last aslong as can possibly be done.

Such filtering screens with asymmetric foldsoffer, therefore, at equalcost price, a longer duration in service than any equivalent screenm'ade of conventional folds.

ll CYLINDRICAL STRUCTURES a. Insulation Structures having asymmetricfolds produced by making use of sheets of insulating materials such aspapers, filters, animal, mineral or vegetable fiber fabrics and the likeand,.in a numbero'f special applications, metals under the formof thin.or very thin sheets, offer easymeans of providing'low cost andefficient insulating coatings for wrapping tubings, or any cylindri apleated filtering layer around a cylindrical free space Anotheradvantage of such cartridges is that the out: side appearance of theirexternal pleats (the wider pleats) is generally more satisfactory thanthat of conventional cartridges, becausev these folds lie on itsperiphery evenly spread in close and identical position. On the other,hand, the practical tests made in submitting cartridges as per theinvention to the passage of the flow of contaminated fluid have shownthat the duration in service is substantially longer than that ofconventional cartridges having the same quality and equal surface offiltering layer pleated in an equal total space.v

The longer life of cartridges made of asymmetric folds results from themanufacturing process which allows for gently pleating the layer withoutcrushing and most often somewhat extending its texture, the uniformspreading of the walls of the folds insuring an even contamination ofthe layer on all its extension.

Further objects and advantages of the present invention will become moreapparent in connection with the ensuing description and claims andappended drawings wherein:

FIG. 1 is, in principle, the outline of the longitudinal profile of thefolds of any structure made as per the invention;

FIG. 2 is a perspective view of a structure as per the invention issuedby simple rectilinear pleating of a flat sheet of drawable or extensiblematerial;

FIG. 3 is a perspective view of a structure produced by simplerectilinearpleating of a longitudinally corrugated sheet of drawable orextensible material;

FIG. 4 is a perspective view of a structure produced by chevron pleatingof a flat sheet of drawable or extensible material;

FIG. 5 is an enlarged sectional view of the three-ply sandwich which isused for fashioning the structures as per the invention as it appearsafter pleating;

FIG. 6 is a longitudinal sectional view of the three plies of thefashioning sandwich showing their respective positions when their foldsare disengaged from one another under the effect of a force imposed onthe extremity of the median layer;

' FIG. is a perspective view of a part of a filtering screen of astructure of the FIG. 4 type;

FIG. .8 is a perspective view of a part of a sandwich structure having acore made of a structure as per the invention and whose covering topplate has been partly or a perforated tube is greatly improved when madeof asymmetric folds (either rectilinear or chevroned) in the manner ofthe filtering cartridge described in US. Pat. Nos. 2,897,971 and3,087,623,the disclosures of which are hereby incorporated herein byreference.

It is advisable, first of all, to mention that making use ofasymmetrical folds leads to an, easier and more economical fabricationof such cartridges. This results from the process for fashioning suchstructure in pairs removed to show the core;

FIG. 9 is a diametrical cross section of a filtering cartridgeconsisting of an asymmetric structure as per the invention;

FIG. .10 is an enlarged view of the innerfolds of the cartridge shown inFIG.-9;- I

FIG. 1 l is an external elevation view of the cartridge of FIG. 9;

FIGS. 12 and 13 are, respectively, elevation and plan views of a flatstructure having asymmetric folds and which is also chevroned in anundulated form;

FIG. 14 illustrates (in elevation on its right side and in section onits left side) a filtering cartridge which has been formed in the shapeof an annulus having an inner axial free space; and

FIG. 15 illustrates (partly in plan view and partly in section) thefiltering cartridge shown in FIG. 14.

The outline drawn in FIG. 1 clearly shows the essential characteristicof structures with asymmetric folds made in accordance with thetechnique of the present invention. As can be seen in this figure, insuch a structure, consisting of the alternate pleating of a sheet ofdrawable or extensible material, the folds a, all identical to oneanother, which are located on one of the two main faces of saidstructure, have a profile whose encumbrance is somewhat larger than thatof folds b, also all identical to one another and on the opposite face.This outline is correct in its principle for any structure as per theinvention, should the folds be rectilinear or chevroned.

FIG. 2 is a materialization, under the shape of a structure havingrectilinear folds c, of the outline shown in FIG. 1, made out of a flatsheet of drawable material.

FIG. 3 is another materialization of the same outline which is also madein the form of astructure having rectilinear folds d, but in making useof a sheet of drawable material which, prior to the operation, has beencorrugated lengthwise, that is to say, corrugated in a general directionperpendicular to that of the folds d.

FIG. 4 is a materialization of an outline similar to that of FIG. 1which, in making use of a flat sheet of drawable material, is producedunder the shape of a structure having chevroned folds e.

'As has been explained hereinbefore, a practical process for fashioningan asymmetric structure of any of the types described as per theinvention is to fold together two sheets of drawable or extensiblematerial f and g (FIG. 5) between which is inserted a layer h ofmaterial having suitable thickness and pliability permitting it to yieldto numerous successive foldings without showing excessive wear orpermanent malformation.

The sectional view of FIG. 5 shows after pleating a three-ply sandwichset up in this way. By reason of the insertion of the median layer hbetween the two sheets f and g, the external folds a of each of saidsheets have a larger encumbrance than that of the internal folds b, thisleading to theformation of two structures showing asymmetric folds andhaving the essential characteristics of the invention.

FIG. 6 clearly shows how, by pulling the extremity of the median layerIt in the direction of arrows F, the two structures folded as above andmaking use of two sheets f and g disengage their mated folds withoutalteration of their shape.

FIG. 7 shows a broken piece of filtering layer, part of a flat filteringscreen, which consists of a structure as per the invention identical tothat shown in FIG. 4 and having chevroned asymmetric folds.

Arrows F and F respectively point to the upstream side of the flow ofcontaminated fluid to be cleaned and the downstream flow of the purifiedfluid after passage through the layer. Folds b of smaller encumbranceare located on the upstream face of the layer and folds a of largerencumbrance are on the downstream face. As

shown, the filtering screen has already been in service for anappreciable length of time for the cleaning of air flowing in thedirection of arrows F and F It can be seen that contaminants i,accumulated at the bottom of the larger recesses a have already filledapproximately half their height between their walls and that passage ofair through the layer is now restricted to the part of these walls whichis still free of contaminants. The narrow downstream spaces existingbetween the wider folds are, even at this state of clogging, largeenough for leaving free passage to the purified air, taking into accountthe spaces of triangular section j existing, to this effect, at theangles of the chevroned pleats.

FIG. 8, which represents a broken piece of a sandwich material whosecore consists of a chevroned structure also of the type shownin FIG. 4,shows the spacing regularity of the folds of such structure whichresults from .the character of the invention. As has been previouslyunderlined, the wider folds a, located at the base of the core, arenecessarily of a rounded profile. This insures an intimate contact offolds onto the internal faces of either base k or top plate 1.

FIGS. 9 to 11 are related to a liquid filtering cartridge made of achevroned asymmetric structure as per the invention, wrapped into acylindrical annulus. This wrapping has been accomplished by proceedingas described hereinbefore, by lifting up the two extremities of a fl'atasymmetric structure keeping its wider folds in close contact, untilthey meet, and can be glued together after proper intermeshing. Thelength of the structure as well as the respective encumbrance of theirwider and thinner folds can be computed so that the complete annularcylinder which is to beformed will have the outside and the insidediameters corresponding to those of the cartridge to be made and alsowill have their thinner and wider folds in joining position respectivelyover the internal and external surfaces of said cylindrical annulus.

FIG. 9 shows the position of the thinner internal folds b. As can beclearly seen in the enlarged view of FIG. 10, it should be noted thatsmall spaces of triangular section m exist between adjoining folds muchin the same form as those of a conventional type of chevroned cartridgeand they offer passage to the liquid much in the same way as takes placein such cartridges.

FIG. 11 shows the very remarkable external appearance offered by afiltering cartridge formed from an asymmetric structure made inaccordance with the invention. The fact that the external a folds aremore encumbrant and joined together does not stop the fluid to befiltered to find its way between the external faces of their wallssince, on the one hand, triangular passages similar to those describedhereinbefore exist between them and, on the other hand, since theslightest value of pressure drop through a layer of these folds has forits effect to enlarge, as much as necessary, a wider passage than theone existing through the triangular spaces located on the periphery ofthe central inner free space of the cartridge.

Besides, at the end of the life of such cartridge, it is understandablethat the external folds a have a flattened surface, the walls of eachfold having come into contact under the effect of the pressure dropseriously raised by reason of the decrease of the available openedsurface caused by the clogging filtering layer. This flattening of theexternal folds is the visible sign of the progressive clogging of thefiltering layer.

The cartridge is, naturally, provided at its two ends with sealed caps nand 0 which, in the example shown, both have a central opening inrelation with the central free space of the cartridge. Alternatively,only one of such ends need have such opening, depending on the kind ofpositioning that this cartridge has to satisfy within the inside of thefilter.

The flat structure with asymmetric chevroned folds a and b which isshown in FIGS. 12 and 13 is, in its general constitution, entirelysimilar to those which are shown in FIGS. 4, 7 and 8 in that it has beenmade by chevroning an ensemble of three sheets in the manner shown inFIGS. and 6. It differs from such chevroned structures by thelongitudinal form of the chevroned folds e, which, in the case of thestructure of FIGS. 12 and 13 is undulated and in the form of a smoothcurve instead of being in the form of broken or zig-zag lines withabrupt direction changes.

The longitudinally undulated folds have, of course (if compared to thezig-zag folds), the characteristics of having no break or sudden changein direction. The latter present shaping difficulties primarily when thesheets to be fashioned are of a low resistance material such as arefiltering layers in thin sheets or most plastic sheetings after theyhave been softened by heat in order to ease their forming, or such as isthe case when chevroning is mechanically performed by means of shapingbars as is taught in copending application Ser. No. 514,171. Such bars,when in zig-zag form, have angular projections which will occasionallyproduce perforations in the sheets coming in contact over their surface.The undulated form, which permits the use of shaping bars void ofangular projections, makes practicable the safe chevroning of the weakerqualities of materials.

In practice, structures with asymmetric folds having longitudinallyundulated chevroning offer, for every one of the applications previouslylisted, the same qualities as those having a zig-zag form, this being sofor sandwich materials and filtering screens, as well as for filteringcartridges or insulating products.

The filtering cartridge shown in FIGS. 14 and 15 differ from thecartridge shown in FIGS. 9, l0 and 11 but only by the undulated form ofits internal and external pleats. In this particular case, the smallspaces having triangular sections m, most particularly appearing inenlarged form in FIG. 10, are spaces whose section has the shape of acrescent m, of similar importance in value, nested between a largeexternal curb of one fold and a smaller internal'curb of the followingfold, as can be seen'in FIG. 14. In spite of this difference in shape,

these spaces m (which are materially built and therefor cannot beobstructed) play exactly the same technical function of dischargingcanals for the filtered liquidor fluid as. the triangular spaces m incartridges made of broken folds (as in FIGS. 9-11).

In practice, the filtering cartridges formed of such undulated foldsoffer efficiency, high output, security in service and a cost pricelevel in production which are, all other conditions beingequal,.identical in every respect to those related to cartridges havingbroken chevroned folds.

In the practice of the folding or'bending technique described previouslyin connection with FIGS. 5 and 6, the median layer h which is interposedbetween the external extensible sheets f and g is fabricated from amaterial which is not only sufficiently pliable to permit it to bereused in successive folding operations but which has a sufficientmemory characteristic to revert substantially completely back to itsoriginal planar configuration after each bending operation and which hasdimensional stability under the moderate tension imparted to such medianlayer during its separation from the folded external sheets f and g. Inaddition, such median layer h should be substantially incompressible sothat the desired folds are made, the extensible sheets f and 3 will beappropriately stretched at externally located peaks 0. Suitable examplesof materials appropriate for median layer h are loose feltered fabrics(viz., a pliable felt which is formed of loosely connected fibers),rubber or plastic porous sheets. A particularly satisfactory mediansheet may be formed of woven nylon fabric coated on one of its two faceswith a rubber foam. Cotton orwool velvet sheets are also suitable.

The thickness of the median layer'h will necessarily vary depending uponthe amount of space desired between the walls of the external sheets fand g which join together to form the wide peaks a. The thicker themedian layer, the greater the amount of stretching of the externalsheets f and g which will take place and the wider the space which willbe formed between such sheet walls. An appropriate thickness of themedian layer h for annular filter cartridge applications has beenone-eighth inch.

The external extensible sheets f and g may be formed of a great varietyof sheet materials provided only that such materials are sufficientlymalleable or pliable to permit them to be bent appropriately during theforming process and also sufficiently drawable or extensible to permitthem to be stretched adequately at each peak a so that they will beconverted to the desired undulated form. Such materials include ordinarywriting and kraft papers but preferably those which are porous innature; filtering papers; felts, preferably of loose constitutionand'particularly those of the filtering feltered variety; loose wovenfabrics of animal, vegetable or artificial fibers such as cotton, wool,silk, nylon, etc.; thermoplastic materials such as cellulose acetate,cellulose acetate butyrate, polyvinyl chloride, polyethylene,polystyrene, etc., as well as woven fabrics made from fibers of suchmaterials.

In order to .facilitate the formation of the folds, it is generallyadvisable to increase the pliability and extensibility of the externalsheets f and g at the time they are formed. This can be effected byslightly steaming or dampening the sheets (such as when paper isemployed) or by heating them (as in the case of thermoplastic materials)during their upstream travel on the forming machine. Similarly, porousmaterials used for the purpose may be impregnated with a material suchas a melamine or urea-formaldehyde resin which will be permitted to setafter the sheets f and g have been appropriately folded. As will beapparent to those skilled in the art, appropriate drying and settingtechniques may be utilized to improve the retention by the foldedexternal sheets of the folds imparted to them by the foregoingprocedure.

The thickness of external sheets f and 3 may vary between wide limitsdepending upon their ultimate end use. For filter cartridgeapplications, filter material having a thickness of up to one-fourthinch has been found to be appropriate.

The forming technique described in connection with FIGS. 5 and 6employed three sheets sandwiched together, the two external sheets f and3 being folded into the desired configuration and the central or mediansheet h serving as the forming layer. This three-ply technique providessignificant advantages in that it makes possible the simultaneousformation of two folded sheets. In addition, the use of the three-plysystem facilitates the desired separation of the walls of the foldsbetween wide dimensional limits since such separation can readily bevaried fora given thickness of external sheets f and g by variation inthe thickness of median sheet h.

Notwithstanding the foregoing advantages of the three-ply system,advantages of the present invention are also obtainable with other thanthree-plysystems. For example, instead of two external sheets f and g,only one such sheet need be placed in contact with forming sheet h(albeit in contact with only one side of such sheet), in which case onlyone folded sheet will be formed during a given folding operation. Theelimination of such second external sheet will, of course, necessarilydecrease the overall thickness of the assembly and the separationbetween the walls of the folded sheet for a given sized median sheet hwill be diminished correspondingly. Similarly, more than one extensiblesheet may be placed on each side of the median sheet h so that more thanone folded sheet will be formed on such side of said median sheet.

Still another alternative presented is to eliminate the median sheetaltogether and merely fold a plurality of sheets (even if both are thesame extensible sheets) together as a unit. To the extent that thethickness of such sheets permits, the exposed portions of each sheetcoinciding with peaks a (as seen in FIG. will be stretched to an extentexceeding any extension taking place at peaks b (as seen in FIG. 5) anda folded sheet of asymmetric configuration will result. Quite obviously,this approach is not as advantageous as the system employing thepliable, incompressible median layer h as previously described.

A significant advantage of the folding technique of the presentinvention resides in its utility in forming asymmetrically foldedstructures of the type heretofore 7 described but having corrugationsalong an axis normal to the peaks of such asymmetric folds. More.specifi- I cally, when corrugated material is bent or folded along aline transverse to the direction of the corrugations (assuming that thecorrugated material is either sufficiently pliable to permit its beingso folded or sufficient force is applied to so fold a structure whichwould otherwise be too rigid to permit such folding), it is notpracticable with presently known techniques to impart a rounded profile(similar to the profile of peaks a in the structure of FIG. 5) to thematerial along the axis of bending. Unless special tooling is used, asubstantially straight line fold forming a sharp angle will result. Andeven with such special tooling, the best that could be expected would bea flattening of .the corrugations in the area adjacent the axis ofbending.

In conventional annular filter cartridges, with the filter annulus beingarranged in star-shaped configuration about the central'filter axis, itis desirable to-have the filter material corrugated along radial lineswith the peaks of the corrugations extendingina direction normal to theaxis of the filter annulus. Such corrugations are designed to spreadapart the radially extending walls of the filter material so as tomaintain filtering passages between such walls. Because of thedifficulty of avoiding flattening of such corrugations at the folds ofthe filter annulus (for reasons indicated above), however, it isnecessary in such filter cartridges to undesirably limit the maximumnumber of folds at the inner periphery of the filter annulus to avoidinoperability of the filter due to irregular spreading or collection ofthe filter layers under service conditions.

The forming technique of the present invention, however, eliminates theundesirable flattening of the corrugated filter material during thefolding operation by which the asymmetric structures having roundedprofiles are formed. Because the median spacer sheet is pliable andsubstantially incompressible, it takes the form of the corrugations ofthe external extensible sheets sandwiched about it and the corrugationsare not destroyed along the axes of the folds which are imparted to thelatter sheets. As a result, filter materials made in accordance with thepresent invention from corrugated sheets may be employed to form anannular filter cartridge having a greater concentration of folds alongits inner peripheral surface than would otherwise be possible usingconventional filtering materials.

Asymmetric structures having rounded profiles as described hereinbeforehave still further advantages. For example, when employed as filteringmedia, the rounded profiles of the folded sheet provide greaterresistance to crushing when the direction of fluid flow through thefilter is from the wide peaks a towards the narrower peaks b since therounded profile of peaks a serves as an arch to resist any potentiallycrushing force. On the other hand, when fluid flow is in the oppositedirection, the capacity of the filter is increased due to the increasedarea available to entrain solids (provided by the wider spacing betweenthe walls of the alternate undulations of the filter sheet).

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency ofthe claims are therefore intended to be embraced therein.

Iclaim: v

l. A structure for use as a filter comprising alternate asymmetricalfolds of pliable material, said structure having a succession ofundulations forming a succession of peaks along at least two main facesand walls integrally connecting said peaks; each peak being on anopposite face of said structure from its next adjacent peaks in saidsuccession; all of said peaks being formed along axes that aresubstantially parallel to one another; the peaks along at least one mainface of said structure being substantially identical to the other peaksalong said one main face and having a substantially rounded profile incross-section, the walls of each undulation which are joined together toform said rounded profile peaks at said one main face of said structurebeing more widely separated from one another than the walls of eachundulation which are joined together to form the peaks at the other mainface of said structure so that said structure is asymmetrical; thecurvature of the peaks at said one main face being less than thecurvature of the peaks at said other main face; the separation of thewalls of each undulation which are joined together to form all of saidpeaks being substantially constant throughout the length of each suchpeak; said structure also having a plurality of folds in a directionsubstantially normal to said peak.

2 A filter unit having a filter formed of the structure defined in claim1.

structure being parallel to the longitudinal axis of said 7 annulus;said rounded profile peaks being located adjacent the outer periphery ofsaid annulus with the peaks on said other side of said structure beinglocated adjacent the inner periphery of said annulus.

6. A structure as defined in claim 1 wherein said plurality of foldsform part of a herringbone configuration; the peaks of said structure atsaid other side of said structure being sharply creased.

7. A structure as defined in claim 6 wherein said peaks of saidstructure at said other side of said structure are undulating in shapein the form of a smooth curve without any abrupt direction changes.

8. A structure as defined in claim 1 wherein the thickness of said sheetof material forming vsaid rounded profile peaks is less than that of thethickness of said sheet forming the peaks atthe otherside of saidstructure.

9. A sandwich structure comprising a low density core and an outer highdensity skin bonded to each face of said core; said core beingconstituted by the structure of claim 1; said rounded profile peaksbeing in contact with and bonded to one of said outer skins; the peaksat said other side of said structure being in contact with and bonded tothe other of said outer skins.

1. A structure for use as a filter comprising alternate asymmetricalfolds of pliable material, said structure having a succession ofundulations forming a succession of peaks along at least two main facesand walls integrally connecting said peaks; each peak being on anopposite face of said structure from its next adjacent peaks in saidsuccession; all of said peaks being formed along axes that aresubstantially parallel to one another; the peaks along at least one mainface of said structure being substantially identical to the other peaksalong said one main face and having a substantially rounded profile incross-section, the walls of each undulation which are joined together toform said rounded profile peaks at said one main face of said structurebeing more widely separated from one another than the walls of eachundulation which are joined together to form the peaks at the other mainface of said structure so that said structure is asymmetrical; thecurvature of the peaks at said one main face being less than thecurvature of the peaks at said other main face; the separation of thewalls of each undulation which are joined together to form all of saidpeaks being substantiallY constant throughout the length of each suchpeak; said structure also having a plurality of folds in a directionsubstantially normal to said peak.
 2. A filter unit having a filterformed of the structure defined in claim
 1. 3. A filter unit as definedin claim 2 wherein the side of said structure containing said roundedprofile peaks is positioned downstream of the flow of the medium to befiltered.
 4. A filter unit as defined in claim 2 wherein the side ofsaid structure containing said rounded profile peaks is positionedupstream of the flow of the medium to be filtered.
 5. A filter unithaving a filter formed of the structure defined in claim 1; saidstructure being formed in the shape of a continuous annulus with thepeaks of said structure being parallel to the longitudinal axis of saidannulus; said rounded profile peaks being located adjacent the outerperiphery of said annulus with the peaks on said other side of saidstructure being located adjacent the inner periphery of said annulus. 6.A structure as defined in claim 1 wherein said plurality of folds formpart of a herringbone configuration; the peaks of said structure at saidother side of said structure being sharply creased.
 7. A structure asdefined in claim 6 wherein said peaks of said structure at said otherside of said structure are undulating in shape in the form of a smoothcurve without any abrupt direction changes.
 8. A structure as defined inclaim 1 wherein the thickness of said sheet of material forming saidrounded profile peaks is less than that of the thickness of said sheetforming the peaks at the other side of said structure.
 9. A sandwichstructure comprising a low density core and an outer high density skinbonded to each face of said core; said core being constituted by thestructure of claim 1; said rounded profile peaks being in contact withand bonded to one of said outer skins; the peaks at said other side ofsaid structure being in contact with and bonded to the other of saidouter skins.